Recently, at information business sites such as data centers, it is becoming more and more important to reduce the total owning cost of systems. In order to reduce the total owning cost, adoption of a virtual machine (hereinafter referred to as the “VM”) technology is being considered. An example of the VM technology is described in U.S. Pat. No. 6,397,242B.
With the VM technology, the server software runs a virtual CPU called a VM. Generally, the use environment of the server is not always a maximum load. Therefore, it is suggested that one physical server runs multiple VMs which are virtual servers. Thus the number of installed physical servers can be reduced, and the use of VMs can reduce, in addition to the installation cost, costs for the maintenance and management of servers and the power consumption of data centers.
However, since the OS (operating system) runs independently in each VM, the memory size required by each OS is the same in the operation of the physical server and in the operation of the VMs. Therefore, if multiple VMs run on a physical server whose main memory (physical memory) capacity is limited, the insufficiency of the memory capacity becomes significant. Meanwhile, the technology of providing memory capacity among multiple VMs is, for example, described in US2006/0136653A.
Furthermore, virtual memory is a measure for cases where programs require more memory size than the capacity of the physical memory. With the virtual memory technology, by using secondary storages such as hard disks and those including disk arrays, physical memory areas are allocated to the secondary storage, which makes it possible for the higher-level device to recognize a larger memory space than the physical memory capacity. Virtual memory includes, for example, those described in Japanese Patent Publication No. 2008-276326A and No. 2005-309739A.
Note that the No. 2005-309739A describes a storage system which a large-capacity cache memory and, by using a high-speed elemental device such as a DRAM, whose latency is several thousand times faster than secondary storage devices such as hard disk drives (HDDs).